engleski

Rejuvenation phenomena of proton exchange membrane fuel cells performance

Degradation (i.e. voltage loss over time) of proton exchange membrane (PEM) fuel cell performance mainly refers to irreversible (also called permanent or unrecoverable) performance decay, which could be caused by several degradation mechanisms on various fuel cell components. However, when the fuel cell exploitation or its durability testing is intentionally or unintentionally interrupted, such as equipment maintenance, start-up/shutdown procedure, overnight rest, continuous electrochemical in situ testing, etc., a phenomenon of performance recovery could occur. Therefore, some degradation may be recoverable, also called temporary, reversible, regenerative, restorable, or transient performance degradation. The main reason for this recovery or rejuvenation phenomenon is often a result of transient processes, commonly attributed to non-ideal inner water and heat management, i.e. changes in the water balance of the fuel cell, where the loss in voltage may be reversed completely or partially by changing the operating conditions (or with the aid of some special in situ recovery procedure), and consequently the cell performance may return or at least it may come closer to pre-degraded levels. Therefore, the recoverable activity loss in diagnostics should be eliminated from the overall performance degradation or at least minimized in order to correctly evaluate the true performance degradation and to prevent premature failure due to the irreversible degradation. Unfortunately, the most accelerated stress test (AST) protocols, have no descriptions/consideration of the reversible losses or appropriate procedure to eliminate or recover the reversible losses. The goal of this study is to present the impact of shutdown procedure and duration of the resting period on PEM fuel cell rejuvenation. An already conditioned standard 50 cm2 (single) fuel cell was exposed to AST protocol consisting of voltage cycling, designed to target electrocatalyst degradation, but with the intentional recovery periods (so-called soak time steps) every 2500 voltage cycles. Before and after every intentional soak time, a series of diagnostic methods (namely polarization curves, electrochemical impedance spectroscopy, cyclic voltammetry, linear sweep voltammetry) were performed. Different shutdown procedures, as well as different duration of the soak time period were tested and their impact on performance recovery evaluated. The results suggest that cause of the reversible degradation could be accumulated water within the cell and/or presence of oxygen within the catalyst layer leading to formation of Pt oxides on the catalyst surface. The prolonged soak time step reduces recovery effect, while rapid reduction of the cell temperature with ice proved to be counterproductive for performance recovery. Shutdown procedure without shortly-connected resistor has shown no effect on recovery. Shutdown procedure without nitrogen purge proved to be the most effective for performance recovery.

Fuel Cells ; Proton Exchange Membranes (PEM) ; Recovery ; Regeneration ; Reversible Degradation

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